This invention relates to compression release mechanisms for internal combustion engines.
It is often desirable to relieve the pressure in an engine combustion chamber during starting so that it is easier for the piston to reciprocate in the engine and thus easier for the operator to manually pull the starter rope. Known compression release mechanisms lessen the pull force required to start the engine, and minimize operator fatigue during starting.
One typical compression release mechanism is disclosed in U.S. Pat. No. 3,381,676 issued May 7, 1968 to Campen. The Campen compression release mechanism includes a centrifugally-responsive flyweight, a torsional spring attached to the flyweight, and a central pin which engages a valve tappet at engine starting speeds. At higher engine speeds, the flyweight moves radially outwardly so that the pin disengages the valve tappet when the engine is running.
It is known to use a compression release mechanism for multi-cylinder engines. For example, U.S. Pat. No. 5,809,958 issued Sep. 22, 1998 to Gracyalny discloses a centrifugally-responsive flyweight to which is connected a compression release shaft disposed externally of the camshaft. The compression release shaft is connected at one end to the flyweight and extends through respective bores in two cams lobes. The release shaft includes two D-shaped cross-sectional portions which engage two respective lift members. One disadvantage of such an arrangement is that the bores for the release shaft must be drilled subsequently to heat treating the cams. Consequently, the drilling operation is more difficult, time consuming and expensive because the heat treated cams are much harder. Another disadvantage of such an arrangement is that the drilling operation is more difficult in that two separate bores must be drilled. This introduces the possibility of mislocating the bores with respect to one another. Another disadvantage of such an arrangement is that the release shaft is supported by a minimum bearing surface, viz., the two bores in the cams. Consequently, the material from which the release shaft is made must be sufficiently strong.
Japanese No. 2-67409(A) to Yoshiharu Isaka also discloses a compression release mechanism for use with multiple cylinders. A flyweight is disposed on the internal side of the cam gear and has a compression release shaft connected thereto. The compression release shaft is disposed internally of the camshaft and includes two D-shaped cross sectional portions therealong, each of which engages a separate lift member, which in turn engage separate valve tappets.
It is desirable to further reduce the cost and at the same time, simplify the assembly of a compression release mechanism.